1. Field of the Invention
The present invention relates to compliant spring contacts fabricated using photolithographic techniques. More specifically, the present invention relates to compliant spring contacts which may be used in electrically connecting integrated circuits to another external electrical device.
2. State of the Art
Semiconductor devices including integrated circuitry, such as memory dice, are mass produced by fabricating hundreds or even thousands of identical circuit patterns on a single semiconductor wafer or other bulk semiconductor substrate using photolithography in combination with various other material deposition and removal processes. These semiconductor devices are subsequently electrically connected to other electrical components, such as additional semiconductor devices, printed circuit boards (PCBs), and probe cards, among many others.
Electrical contact structures are an integral part of connecting semiconductor devices to external electrical components, such as other semiconductor devices, PCBs, probe cards, etc. There are several standard bonding methods known in the art for electrically connecting semiconductor devices to another electrical device. Some of these methods, as illustrated in FIG. 9, include wire bonding, tab bonding, solder-bump bonding, and flip-chip bonding, among many other methods. Referring to FIG. 9, a semiconductor chip 28a having a ball grid array 30 is flip-chip bonded to a printed circuit board (PCB) 32. Semiconductor chip 28b is bonded to PCB 32 using an adhesive 40 and in electrical communication with PCB 32 using wire bonds 38. Tab bonding is illustrated with semiconductor chip 28c bonded to PCB 32 using an adhesive 40 and in electrical communication with PCB 32 using tape leads 36. While the above methods have adequately worked in the past, the trend in semiconductor devices is to use smaller and smaller electrical contacts to accommodate the greater number of circuits on the substrates which pose difficulties for the above mentioned conventional bonding methods.
An example of fine pitch electrical contacts is described in U.S. Pat. No. 5,848,685 to Smith et al. (“the Smith Patent”) entitled “Photolithographically Patterned Spring Contact” the disclosure of which is herein incorporated by reference and the article in Advanced Packaging, Linder at al., “Nanosprings—New Dimensions in Sputtering,” pp.44-47. The above references disclose photolithographically patterned spring contacts that may be used for flip-chip contacts or for probe card applications. FIG. 10 is a side view of such a spring contact. Bonding structure 100 includes a plurality of spring contacts 34. Each spring contact 34 comprises a free, cantilevered portion 42 and an anchor portion 46 fixed to an insulating underlayer 48 made from silicon nitride or other etchable insulating material and electrically connected to a contact pad 50. Each spring contact 34 is made of an extremely elastic material, such as a chrome-molybdenum alloy or a nickel-zirconium alloy. The contact pad 50 is the terminal end of a line or trace which electrically communicates between an electronic device formed on the substrate 44 or device 101 such as a transistor, a display electrode, or other electrical device. However, while the patterned spring contacts of the Smith Patent may be used to create a fine array of spring contacts connectable to an external device, it suffers from several deficiencies.
First, Smith discloses using extremely elastic materials for the spring contacts. Examples of extremely elastic materials disclosed in Smith are chrome-molybdenum alloys or nickel-zirconium alloys, both of which are poor electrical conductors compared to traditional electrical contact materials made predominately from copper or aluminum. Second, the entire spring contact is formed from the stressable material. Third, stresses from the films used are not localized. Instead, the entire film used for the spring contact is under stress. Finally, in practice, when the spring contacts of Smith are soldered to contact pads, the entire spring contact becomes covered with an excessive amount of solder material, resulting in a lack of flexibility of the spring contact, preventing the spring contact from deflecting toward the substrate. This results in fracture of the spring contact when loaded.
Accordingly, in order to improve the performance of spring contacts, a need exists for a conductive spring contact in which conventional thin film fabrication techniques may be used. A need also exists to create a localized stressed region in the thin films. Furthermore, a spring contact is needed which is durable when loaded.